1. Field
The present disclosure generally relates to the fields of polymer materials and gas separation membranes. More specifically, it relates to the polymer blends, which are immiscible before the addition of a compatibilizing agent that are useful for the separation of two or more different gas molecules.
2. Related Art
Membrane technology has become a promising alternative to conventional energy intensive gas separation methods. Membranes are being used commercially, for H2 recovery, CO2 recovery from natural gas and onsite nitrogen production from air (Bernardo et al., 2009; Baker and Lokhandwala, 2008; Baker, 2002; Ma et al., 2013). However, the selectivity/permeability tradeoff of polymer based membranes in gas separations, shown by Robeson (Robeson, 2008; Robeson, 1991), has motivated research to develop membranes that surpass the upper bound (FIG. 1).
Despite the many efforts that have been made to reach the upper bound, including synthesis of new polymers (Sakaguchi et al., 2005; Lively et al., 2012; Sanders et al., 2012; Park et al.), cross-linking of the polymers (Qiuj et al., 2011; Ribeiro et al., 2011; Wijenayake et al., 2013), fabrication of inorganic-organic composite materials (mixed-matrix membranes) (Ordonez et al., 2010; Bae et al., 2010; Perez et al., 2009; Chung et al., 2007), use of polymer blends (Mannan, et al., 2013), and use of carbon molecular sieve membranes (CMSM) (Park et al., 2007; Ning and Koros, 2014; Rungta et al., 2012; Low et al., 2011; Paul, 2012), very few systems have emerged that exceed this now two-decade old limit.
On the other hand, polymer blends of polyimides (PIs) are attractive, since blending can synergistically combine favorable properties of polymers such as high selectivity and high permeability, which cannot be attained with a single polymer (Hosseini et al., 2008). Also, polymer blending is less time consuming and cost effective approach compared to the development of new materials and further it allows changing properties like gas permeability by varying the blend composition. However, due to the unfavorable thermodynamics of mixing (Robeson, 2007; Isayev, 2010), polymers can phase separate and requires compatibilizer to obtain uniform morphologies.
Generally, copolymers (Semsarzadeh et al., 2011) and nanoparticles (Fenouillot et al., 2009; Goodarzi, et al., 2013) are used to compatibilize immiscible polymer blends to obtain uniform microstructures. However, the synthesis of copolymers is tedious and applicability is limited to a single blend (U.S. Pat. No. 6,339,121). Nanoparticles also need synthesis and sometimes chemical modifications (Chung et al., 2012). Given the need for overcoming these obstacles to produce compatibilized immiscible polymer blends, polymers prepared using other compatibilization methods are needed.
Additionally, the polymer blends may be used in the preparation of a gas separation membrane. One important aspect of determining the performance of an immiscible polymer blend based membrane is the blend's morphology (Robeson, 2010). The matrix-droplet type morphology is preferred for membrane transportation over the others, since it can provide a large interfacial area to enhance flux. 6FDA based PI polymers have high gas permeabilities due to high free volume (Suzuki, et al., 2004) while PBI polymers have high selectivity (Yang, et al., 2012). The present disclosure provides the use of small molecules and metal organic frameworks (MOFs) to compatibilize immiscible blends of polybenzimidazole polymers and 6FDA-based co-polyimide (9FDD) polymers to form membranes with a uniform and novel microstructure to improve gas separation performances. These compatibilized immiscible blends may have one or more advantages including the use of small molecules which are commercially available, inexpensive, and easy to use.
By combining both polymers, the resulting membranes possess a unique microstructure containing thin, continuous ribbons in an otherwise noncontinuous polymer matrix. The dispersed, highly permeable phase occupies most of the effective area/volume (dispersed phase) to achieve high flux (FIG. 2) while the continuous phase is composed of PBI to ensure higher selectivity.